A high-strength machine component having a tensile-strength of 900 MPa or more is used for a vehicle and various industrial machineries to reduce weight and size thereof. Conventionally, this kind of high-strength machine component is manufactured by using steel materials of an alloy steel and a special steel in which alloying elements such as Mn, Cr, Mo, or B are added to a carbon steel for machine structural use, performing spheroidizing annealing after hot-rolling to soften the material, forming into a predetermined shape by performing cold forging and form rolling, and thereafter, supplying strength by performing a quench-hardening and tempering process.
However, a steel cost of these steel materials is high because the alloying elements are contained, and a manufacturing cost thereof increases because soften annealing before it is formed into a component shape and the quench-hardening and tempering process after the forming are required.
An art is known in which wire drawing is performed for a wire material whose strength is increased by quick cooling, precipitation strengthening, and so on without performing the soften annealing and the quench-hardening and tempering process to supply a predetermined strength. This art is used for a bolt and so on, and the bolt manufactured by using this art is called as a non-heat treated bolt.
In Patent Document 1, a manufacturing method of the non-heat treated bolt is disclosed in which a wire material containing C: 0.15% to 0.30%, Si: 0.03% to 0.55%, Mn: 1.1% to 2.0% is cooled in a boiling water bath, and a drawing process is performed with a reduction of area of 20% to 50%. In this manufacturing method, it is possible to omit the spheroidizing annealing and the quench-hardening and tempering process, but a maximum strength of the bolt described in the example is 88 kgf/mm2, and it cannot be said that this bolt has enough strength, and there is a limit in high-strengthening.
In Patent Document 2, a cold forging steel containing C: 0.4% to 1.0%, whose chemical composition satisfies a specific conditional expression, and whose structure is made up of pearlite and pseudo pearlite is disclosed. A C amount of this steel is large, and cold forgeability thereof deteriorates compared to a carbon steel for machine structural use and an alloy steel for machine structural use which are conventionally used for machine components such as a bolt.
As stated above, a machine component having good cold forgeability and a strength of 900 MPa or more and a steel wire and a wire material to manufacture the above cannot be obtained by non-heat treated wire materials according to the conventional arts.